Beneficial effect of moderate food restriction in toxicity studies in rats.

Moderate food restriction (FR) has been established as a nutritionally appropriate and well-controlled method with long-term beneficial effects in conducting toxicity and carcinogenicity studies in rodents. This study describes the early effects of moderate FR on toxicity study parameters in rats and on the variability of these parameters. Physical signs, body weight, food and water consumption, and clinical pathology parameters were examined in a 4-week study in which rats were moderately food-restricted or fed ad libitum (AL). There were no diet-related differences in physical signs, hematology or urinalysis. FR-related changes were observed in body weight and serum biochemistry; however, most of the changes involved anti-aging alterations and/or physiological adjustment to FR. Moderate FR resulted in low variability and good reproducibility in body weight. The present results indicate that moderate FR does not impair study parameters and increases statistical sensitivity. Therefore, a moderate FR feeding regimen is beneficial not only for long-term but also for short-term toxicity studies in rats.

Glutathione secretion into rat milk and its subsequent gamma-glutamyltranspeptidase-mediated catabolism.

Reduced glutathione (GSH) is a critical cellular antioxidant and a cofactor for enzymes that detoxify carcinogens. GSH is present at high concentrations in all mammalian tissues, including mammary gland tissue (1- 2 mM), and recent studies indicate that the tripeptide may also be present in milk. The present study examined the hypothesis that GSH is secreted into rat milk, and that some of the secreted GSH is degraded by the ectoenzyme gamma-glutamyltranspeptidase (GGT) within the milk space. To test this possibility, milk was collected over a 4-hour period in anesthetized, 9- to 11-day postpartum Wistar rats, as well as in similar rats treated with the GGT inhibitor acivicin. GGT enzymatic activity was relatively high in rat mammary tissue, and acivicin was able to inhibit most ( approximately 80%), but not all of this activity. In control animals, GSH was present in milk, although at a relatively low concentration of approximately 20-25 microM. However, after treatment with acivicin, milk GSH concentration increased 3- to 4-fold. Most of the glutathione was in the reduced form, as no GSSG was detected (<2 microM) in any of the milk samples. These results demonstrate that GSH is secreted into rat milk, but that a large fraction of the secreted GSH is degraded by the ectoenzyme GGT. Taken together with previous studies indicating the presence of GSH synthetic and catabolic enzymes in mammary tissue, these data suggest that the mammary gland has a functional gamma-glutamyl cycle. This cycle may contribute to the secretion of cysteine into milk, which is a rate-limiting amino acid for neonatal growth and development.